Guards at a state-owned coal firm in the northern province of Quang Ninh were bribed to let thieves steal thousands of tons of coal in February, investigators said Tuesday.Police said Le Khac

Hung, head of the security guard unit at Mao Khe Coal Company had received at least VND100 million (US$5,270) to let a band of theives bring machines into a coal mine area to excavate and steal the energy source.

Mao Khe Coal Company operates under the country's top coal producer, Vietnam National Coal-Mineral Industries Group (Vinacomin).

A police search of Hung's home found some savings books and more than VND700 million in cash.

Thieves broke into a mine operated by Mao Khe Coal Company during Tet in the middle of February, when company officials and workers were on holiday. But authorities kept mum about the incident until late March.

Excavators and different vehicles were brought and operated for days on around two square kilometers of the mine. The company didn't report the losses immediately.

Quang Ninh police have detained two guards and three suspected thieves, and have suggested criminal charges against them.

"We see this case as serious and involving many people," said Do Van Luc, deputy director of Quang Ninh police.

Investigators said they are working to determine if local governments had given a hand to the criminals.

Dozens of guards from the Mao Khe company were detained Monday and Tuesday for interrogation.